When I go camping for a couple days at a time, I leave the truck unlocked so no one breaks the window to get in. I keep my sunglasses, my doggie poo bags, some change, sometimes the tire chains, and that's it. No one has ever stolen anything from me when camping, but I've seen it happen with others. The places I go are pretty remote usually and am gone no more than two days. I've had my vehicle broken into twice in the city. I don't keep stuff in my truck. That is going to change in a few weeks, though, as I will be in my camper. Yay!
I lived in a 6-plex about 15 years ago. One morning as I got up for work, a couple of law enforcement officers were taking statements from all of my neighbors. Turns out someone went from vehicle to vehicle smashing the windows and stealing everything inside. I lost a $20 pair of binoculars and that was it. Ended up getting them back! While they were all probably late for work and stuff and maybe had increased insurance rates, I was off to work without a care in the world.
Now that is different than leaving all of your belongings in your home on wheels, but gives you an idea of how secure a vehicle is and what one could do to maybe mitigate the cost of a break in from a determined individual.